Steel plates prepared through quenching and tempering (hereinafter also referred to as “QT steel plates”) have high strength and high toughness, show satisfactory weldability, and have thereby been widely adopted to welded structures such as bridges, high-rise buildings, ships, and tanks. With increasing sizes of such welded structures in recent years, the QT steel plates should have further higher strengths (for example, tensile strengths of 585 MPa or more).
The steel plates should naturally have satisfactory toughness as fundamental properties as base steels (steel plates) and should have excellent drop weight properties as indices of brittle fracture properties. However, known steel plates do not satisfy requirements in these properties when they are designed to have higher strengths and larger gauges.
There is known a good correlation between the drop weight properties and a high-angle boundary grain size. It is also known that reduction of the high-angle boundary grain size is effective for improving the drop weight properties. The “high-angle boundary grain size” refers to the size of a grain surrounded by a high-angle grain boundary with a difference in crystal orientation of 15° or more.
The reduction of the high-angle boundary grain size is most generally performed by finely dividing austenite grains (gamma grains) during quenching. According to this technique, an element that forms carbonitrides even at high temperatures (e.g., Nb and/or Ti) is added to form carbonitrides, and gamma grains are pinned by the action of the carbonitrides to thereby suppress the growth of gamma grains when the steel is heated and held at high temperatures.
This technique, however, fails to finely divide high-angle boundary grains to such an extent as to improve the drop weight properties sufficiently, although the technique gives finely divided gamma grains to thereby give finely divided packets and blocks after transformation, which packets and blocks act as units of fracture.
Another possible technique for reducing the high-angle boundary grain size is increasing hardenability (quenchability), namely, increasing the driving force of transformation to thereby finely divide packets and blocks after transformation.
However, even when the resulting steel plates have better drop weight properties, they may contrarily have inferior base metal toughness, because such steel plates should have larger gauges corresponding to increased demands of large-sized structures, and large amounts of alloy elements should be added to obtain quenched finely divided microstructures in such thick steel plates.
Independently, for example, Japanese Unexamined Patent Application Publication (JP-A) No. S61 (1986)-276920 proposes another technique for improving the drop weight properties. According to this technique, a high-tension steel plate having a predetermined chemical component composition is cooled from a temperature in the range of Ar3 to (Ar3-60° C.) to an arbitrary temperature in the range of 400° C. to 200° C. at a cooling rate of 10° C./second or more to thereby give satisfactory drop weight properties.
This technique, however, is not adoptable to improvements in drop weight properties and base metal toughness of steel plates, because the technique is directed to relatively thin steel plates, and a cooling rate of 10° C./second or more is difficult to achieve in thick steel plates. Under these circumstances, demands have been made to provide steel plates which can surely have satisfactory drop weight properties and base metal toughness only by controlling the contents of necessary alloy elements.